The present disclosure relates to a sheet loading device that is used in an image forming apparatus such as a copy machine, a facsimile, or a printer and a sheet post-processor that performs post-processing such as a punch hole forming process or a binding process with respect to a sheet such as a paper sheet on which an image has been formed by the image forming apparatus, the sheet post-processor and the image forming apparatus provided with the same.
In an electrophotographic image forming apparatus, a circumferential surface of an image carrier (a photosensitive drum) is irradiated with light based on image information read from an original document image or image information obtained by, for example, transmission from an external device such as a computer, and thus an electrostatic latent image is formed thereon. The electrostatic latent image is supplied with toner from a developing device so that a toner image is formed, which then is transferred on a paper sheet. The paper sheet that has gone through the transfer processing is subjected to fixing processing of the toner image and then is ejected from a sheet ejection portion onto an ejection tray.
In such an image forming apparatus, in order to detect a full load state of paper sheets ejected on the ejection tray, there is known a method in which notice is given of such a full load state at a point in time when the number of sheets ejected has reached a preset loadable number of sheets. Here, in a case where large-size paper sheets are to be ejected, since they are heavier in weight than small-size paper sheets, with durability of the ejection tray taken into consideration, the loadable number of sheets is decreased. It is, therefore, necessary that the loadable number of sheets be set to vary depending on whether small-size paper sheets or large-size paper sheets are used. In a case, however, where a load height is increased as a result of, for example, continuously printing high printing rate images, the ejection tray may be brought to a full load state before the loadable number of sheets is reached, which has been disadvantageous.
As another method for detecting a full load state of paper sheets, there is known a method in which a load detection sensor that detects a load height of paper sheets ejected on an ejection tray is provided, and based on a result of detection by the load detection sensor, a full load state of paper sheets on the ejection tray is detected. As such a load detection sensor, there is widely known a transmission type optical sensor that has a light receiving portion and a light emitting portion in a paper sheet width direction. A large-size paper sheet, however, hardly slides down along a slope of an ejection tray, and thus compared with a small-size paper sheet, an upper surface position thereof tends to vary. Furthermore, in a case of using the transmission type optical sensor, paper sheets can be detected only when loaded to a preset load height. It, therefore, becomes necessary that a load detection sensor for a small-size paper sheet and a load detection sensor for a large-size paper sheet be provided separately, resulting in a cost increase, which has been problematic.
As a solution to the above, there is known a sheet processor in which a distance measuring sensor that detects an uppermost surface of sheets is provided, and in a case where it has been detected a plurality of times continuously that a height of an uppermost surface of loaded sheets is not less than a prescribed height, it is judged that the sheets are in a full load state.
There is also known a sheet processor in which, when a weight of sheets loaded on a sheet loading unit has reached a prescribed weight, it is judged that the sheets are in a full load state, and thus sheet loading is stopped. Specifically, an ascending/descending time of a stack tray (the sheet loading unit) when fully loaded with sheets is stored beforehand, and in a case where an actual ascending time is slower than the ascending time of the stack tray when fully loaded with sheets or an actual descending time is faster than the descending time of the stack tray when fully loaded with sheets, it is judged that sheets loaded on the stack tray are in a full or higher load state.